Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the pressure of the system of interest. It requires less water vapor to attain high relative humidity at low temperatures; more water vapor is required to attain high relative humidity in warm or hot air. (wikipedia).
Does humidity has impact on the amount of airborn pollen? This is a pretty difficult question to answer, as pollendata is mainly available as a daily average and humidity can vary a lot in 24 hours. Both from a meteorologic as a biologic point of view is makes sense that there is a strong correlation. Little research has been done in this field though.
Other influencing aspects are: